System and method for work function reduction and thermionic energy conversion

ABSTRACT

A thermionic energy converter, preferably including an anode and a cathode. An anode of a thermionic energy converter, preferably including an n-type semiconductor, one or more supplemental layers, and an electrical contact. A method for work function reduction and/or thermionic energy conversion, preferably including inputting thermal energy to a thermionic energy converter, illuminating an anode of the thermionic energy converter, thereby preferably reducing a work function of the anode, and extracting electrical power from the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/500,300, filed on 2 May 2017, and U.S. Provisional ApplicationSer. No. 62/595,003, filed on 5 Dec. 2017, each of which is incorporatedin its entirety by this reference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Award NumberARPA-E-DE-AR00000664 awarded by the Advanced Research ProjectsAgency-Energy and under Contract Numbers HR0011-17-P-0003 andW911NF-17-P-0034 awarded by the Defense Advanced Research ProjectsAgency. The government has certain rights in the invention.

TECHNICAL FIELD

This invention relates generally to the thermionic energy conversionfield, and more specifically to a new and useful system and method forwork function reduction in the thermionic energy conversion field.

BACKGROUND

Large anode work functions can limit the power conversion efficiency ofthermionic energy converters. Thus, there is a need in the thermionicenergy conversion field to create a new and useful system and method forwork function reduction.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional view of an embodiment of the system;

FIGS. 2A-2C are cross-sectional views of a first, second, and thirdexample, respectively, of an anode of the system;

FIG. 2D is a cross-sectional view of an example of semiconductor layersof the anode;

FIG. 3A is a schematic representation of the method;

FIG. 3B is a schematic representation of an embodiment of the method;and

FIGS. 4A-4B are schematic representations of a band diagram of anembodiment of an anode, without and with illumination, respectively.

FIGS. 5-7 are representations of a calculated band diagram of a first,second, and third specific example of an anode, respectively, withoutand with illumination.

FIG. 8 is a schematic representation of a band diagram of an embodimentof the system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

1. System.

A thermionic energy conversion system 10 (TEC) preferably includes ananode 100 and a cathode 200 (e.g., as shown in FIG. 1). However, thesystem 10 can additionally or alternatively include any other suitableelements.

The anode 100, cathode 200, and/or other elements of the system caninclude (e.g., be made of) any suitable materials and/or combinations ofmaterials. The materials can include semiconductors, metals, insulators,2D materials (e.g., 2D topological materials, single layer materials,etc.), organic compounds (e.g., polymers, small organic molecules,etc.), and/or any other suitable material types.

The semiconductors can include group IV semiconductors, such as Si, Ge,SiC, and/or alloys thereof; III-V semiconductors, such as GaAs, GaSb,GaP, GaN, AlSb, AlAs, AlP, AlN, InSb, InAs, InP, InN, and/or alloysthereof; II-VI semiconductors, such as ZnTe, ZnSe, ZnS, ZnO, CdSe, CdTe,CdS, MgSe, MgTe, MgS, and/or alloys thereof; and/or any other suitablesemiconductors. The semiconductors can be doped and/or intrinsic. Dopedsemiconductors are preferably doped by low-diffusivity dopants, whichcan minimize dopant migration (e.g., at elevated temperatures). Forexample, n-type Si is preferably doped by P and/or Sb, but canadditionally or alternatively be doped by As and/or any other suitabledopant, and p-type Si is preferably doped by In, but can additionally oralternatively be doped by Ga, Al, B, and/or any other suitable dopant.The semiconductors can be single-crystalline, poly-crystalline,micro-crystalline, amorphous, and/or have any other suitablecrystallinity or mixture thereof (e.g., including micro-crystallineregions surrounded by amorphous regions).

The metals can include alkali metals (e.g., Li, Na, K, Rb, Ce, Fr),alkaline earth metals (e.g., Be, Mg, Ca, Sr, Ba, Ra), transition metals(e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Zr, Nb, Mo, Au, Ru,Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Hg, Ga, Tl, Pb, Bi, Sb, Te, Sm,Tb, Ce, Nd), post-transition metals (e.g., Al, Zn, Ga, Ge, Cd, In, Sn,Sb, Hg, Tl, Pb, Bi, Po, At), metalloids (e.g., B, As, Sb, Te, Po), rareearth elements (e.g., lanthanides, actinides), synthetic elements (e.g.,Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn,Nh, Fl, Mc, Lv, Ts), any other suitable metal elements, and/or anysuitable alloys, compounds, and/or other mixtures of the metal elements.

The insulators can include any suitable insulating (and/or wide-bandgapsemiconducting) materials. For example, insulators can includeinsulating metal and/or semiconductor compounds, such as oxides,nitrides, carbides, oxynitrides, fluorides, borides, and/or any othersuitable compounds.

The 2D materials can include any suitable 2D materials. For example, the2D materials can include graphene, BN, metal dichalcogenides (e.g.,MoS2, MoSe2, etc.), and/or any other suitable materials. However, thesystem can include any other suitable materials.

The elements of the system can include any suitable alloys, compounds,and/or other mixtures of materials (e.g., the materials described above,other suitable materials, etc.), in any suitable arrangements (e.g.;multilayers; superlattices; having microstructural elements such asinclusions, dendrites, lamina, etc.).

1.1 Anode.

The anode 100 functions to collect thermionically emitted electrons(e.g., emitted from the cathode 200). The anode 100 preferably includesone or more semiconductor layers 110, and can optionally include one ormore supplemental layers 105 (e.g., electronic protection layers 120,electronic population control layers 125, electron capture layers 130,optical tuning layers 135, chemical protection layers 140, work functiontuning layers 150, etc.), electrical contacts 160, and/or any othersuitable elements (e.g., as shown in FIGS. 2A-2C).

The anode 100 is preferably substantially planar (e.g., is a flatwafer), but can additionally or alternatively define any suitable shape.Each layer of the anode is preferably a continuous thin film (ormultilayer of thin films). However, one or more of the layers canadditionally or alternatively be discontinuous, patterned (e.g.,laterally), textured, have varying composition (e.g., laterally), and/orhave any other suitable conformation. The anode 100 can optionallyinclude surface textures (and/or inter-layer textures, such as betweenadjacent layers), lateral features, and/or any other suitable features.The layers of the anode 100 preferably define substantially sharpinterfaces (e.g., wherein an interfacial region between the layers islimited to substantially as thin as practical, such as 0, 1-3, 3-10,10-30, or 30-100 atomic layers and/or 0-0.3, 0.3-1, 1-3, 3-10, 10-30, orgreater than 30 nm), but can additionally or alternatively includesubstantial (e.g., defining a thickness such as less than 1, 1-3, 3-10,10-30, 30-100, or 100-300 nm) interfacial regions (e.g., includingregions of mixed and/or non-uniform composition, composition gradientsbetween the layers, etc.) and/or include any other suitable interface.The interfacial regions can be defined between layers, can penetratethrough all or substantially all of one or more layers, and/or have anyother suitable arrangement.

The anode 100 preferably includes a first side and a second sideopposing the first side (e.g., opposing broad faces of a wafer). Theanode 100 preferably defines a superficial-deep axis from the first sideto the anode interior between the first and second sides (e.g., towardthe second side). Elements superficial (e.g., along the superficial-deepaxis) to the semiconductor layers 110, such as the electronic protectionlayers 120, electron capture layers 130, chemical protection layers 140,work function tuning layers 150, electrical contacts 160, etc.)preferably transmit light (e.g., photons with energy greater than thebandgap of one or more of the semiconductors, such as the bulksemiconductor 111). The elements can transmit some, all, orsubstantially all of the light, or can alternatively reflect and/orabsorb all or substantially all incident light. However, the anode 100can include any other suitable elements in any suitable arrangement.

1.1.1 Semiconductor Layers.

The anode 100 preferably includes one or more semiconductor layers 110,which can function (e.g., in cooperation with other elements of theanode) to enable photovoltage-based work function control (e.g., workfunction reduction). In some embodiments, the semiconductor layers 110(and/or other anode elements) are engineered to achieve a large built-involtage (e.g., to maximize the built-in voltage), which can result ingreater photovoltage-based work function reduction. The semiconductorlayers 110 preferably include a bulk semiconductor 111, and canoptionally include one or more additional semiconductor layers (e.g.,reduced-doping layer 112, opposite-type layer 113, carrier blockinglayer 114, etc.), such as shown in FIG. 2D. Such additionalsemiconductor layers can function, for example, to increase the built-involtage of the anode (e.g., thereby enhancing the potential photovoltageeffect, which can result in greater work function reduction), to reduceundesired carrier recombination, to tune anode optical properties (e.g.,increasing above-gap light absorption in the semiconductor near thefirst side, reducing above-gap light absorption in the semiconductornear the second side and/or in non-semiconductor regions of the anode,reducing below-gap light absorption, etc.), and/or perform any othersuitable functions.

The semiconductor layers 110 are preferably arranged adjacent eachother, and the additional semiconductor layers are preferably arrangedsuperficial of the bulk semiconductor 111 (e.g., along thesuperficial-deep axis). Adjacent semiconductor layers can formsemiconductor junctions, which can include homojunctions and/orheterojunctions, isotype junctions (e.g., n-n⁺, n-N) and/or heterotypejunctions (e.g., p-n, p-i-n, P-n, etc.), and/or any other suitablejunction types. Adjacent semiconductor layers 110 preferably formhigh-quality interfaces (e.g., having few electronic defects) with theadjacent semiconductor layer(s). For example, the semiconductor layers110 can be epitaxial. Alternatively, some or all of the semiconductorlayers and/or interfaces can include moderate and/or high densities ofelectronic defects (e.g., to achieve Fermi-level pinning at a desiredenergy level), and/or include any other suitable interface aspects. Eachadditional semiconductor layer can have a thickness less than and/orgreater than a threshold maximum and/or minimum thickness (e.g., 1 mm,100 μm, 10 μm, 1 μm, 100 nm, 10 nm, 1 nm, 0.1 nm, 0.1-1 nm, 1-10 nm,10-100 nm, 100-1000 nm, 1-10 μm, several monolayers, monolayer,sub-monolayer, etc.), or can have any other suitable thickness. Theanode 100 can include any suitable number and variety of semiconductorlayers 110 in any suitable arrangement.

The bulk semiconductor 111 is preferably a high-quality (e.g.,single-crystalline, low-impurity, etc.) semiconductor, but canadditionally or alternatively include semiconductor materials of anysuitable quality. The bulk semiconductor 111 is preferably Si, galliumarsenide (e.g., GaAs), aluminum gallium arsenide (e.g.,Al_(x)Ga_(1-x)As), gallium indium phosphide (e.g., Ga_(x)In_(1-x)P), oraluminum gallium indium phosphide (e.g., Al_(x)Ga_(y)In_(1-x-y)P), butcan additionally or alternatively include any suitable semiconductormaterials (e.g., as described above).

The bulk semiconductor 111 is preferably an n-type semiconductor (e.g.,so that a photovoltage effect caused by illumination of the bulksemiconductor will reduce the anode work function). However, the bulksemiconductor 111 can additionally or alternatively include p-typesemiconductor material, intrinsic semiconductor material, and/or anyother suitable doping(s). The bulk semiconductor 111 is preferablyhighly doped (e.g., equilibrium charge carrier density greater than athreshold level such as 10¹⁵/cm³, 10¹⁶/cm³, 10¹⁷/cm³, 10¹⁸/cm³,10¹⁹/cm³, 10²⁰/cm³, etc.; equilibrium carrier density in the range10¹⁵/cm³-10¹⁶/cm³, in the range 10¹⁶/cm³-10¹⁷/cm³, in the range10¹⁷/cm³-10¹⁸/cm³, in the range 10¹⁸/cm³-10²⁰/cm³, etc.), but canadditionally or alternatively include lower doping (e.g., equilibriumcarrier density less than 10¹⁵/cm³, less than 10¹⁴/cm³, less than10¹²/cm³, in the range 10¹⁴/cm³-10¹⁵/cm³, in the range10¹²/cm³-10¹⁴/cm³, etc.), which may be desirable, for example, to reducefree carrier absorption, and/or any other suitable doping level. In aspecific example, the bulk semiconductor 111 has an equilibrium carrierdensity in the range 10¹⁶/cm³-3×10¹⁷/cm³ (e.g., 1-3×10¹⁶/cm³,3-6×10¹⁶/cm³, 6-10×10¹⁶/cm³, 1-3×10¹⁷/cm³, 7.5×10¹⁶/cm³-2×10¹⁷/cm³,etc.). The bulk semiconductor 111 preferably has substantially uniformdoping, but can additionally or alternatively include doping changes(e.g., changing laterally and/or with depth) such as gradients,discontinuities, and/or any other suitable doping features.

The bulk semiconductor 111 is preferably wafer-thick (e.g., 150 μm-1 mm,50-150 μm, 1-3 mm, etc.), more preferably having a thickness in therange 50-250 μm (e.g., 50 μm, 100 μm, 125 μm, 150 μm, 175 μm, 200 μm,250 μm, etc.), but can alternatively be thicker (e.g., many mm thickslab, greater than 1 cm thick, etc.), thinner (e.g., less than 50 μm,such as a thin film or multilayer), or have any other suitablethickness. In some examples, the bulk semiconductor 111 is thicker thanthe than its hole diffusion length (e.g., greater in thickness by afactor such as 1.1, 1.2, 1.5, 2, 3, 5, 10, 20, 30, 50, 100, 1.01-1.1,1.1-1.3, 1.3-1.5, 1.5-2, 2-3, 3-5, 5-10, 10-30, 30-100, etc.), which canfunction, for example, to reduce Fermi level splitting at the back sideof the semiconductor (e.g., side proximal the anode second side), whichcan otherwise reduce the device output voltage. Additionally oralternatively, a thinner bulk semiconductor may be desirable, forexample, to reduce free carrier absorption, which can otherwisecontribute to parasitic heat transfer (e.g., from the cathode to theanode). The bulk semiconductor 111 preferably functions as a mechanicalsupport for the anode 100 (e.g., supports the anode weight duringhandling and/or operation), but the anode 100 can additionally oralternatively include one or more mechanical support structures (e.g.,substrate, ribs, etc.). For example, the anode 100 can include asemiconductor-on-insulator substrate (e.g., silicon-silica-siliconsubstrate). Although described as a bulk semiconductor, a person skilledin the art will recognize that a semiconductor of any suitable thickness(and/or other dimensions) can be used in the anode as the bulksemiconductor 111.

The reduced-doping layer 112 is preferably doped the same type (e.g.,n-type, p-type) as the bulk semiconductor 111, with a lower equilibriumcarrier density than the bulk semiconductor 111. The reduced-dopinglayer 112 can additionally or alternatively include one or moreintrinsic or minimally doped layers. The reduced-doping layer 112 ispreferably made of the same material as the bulk semiconductor 111(e.g., forming an isotype homojunction with the bulk semiconductor 111),but can additionally or alternatively include different semiconductormaterial(s) than the bulk semiconductor 111 (e.g., forming an isotypeheterojunction with the bulk semiconductor 111). For example, in ananode 100 with a highly-doped (e.g., equilibrium carrier concentrationgreater than 10¹⁸/cm³) n-Si bulk semiconductor 111, the reduced-dopinglayer 112 can be n-Si with a lower doping level (e.g., equilibriumcarrier density less that of the bulk semiconductor 111, less than athreshold value such as 10¹⁶/cm³, etc.).

The reduced-doping layer 112 is preferably arranged adjacent to andsuperficial of (e.g., grown epitaxially from) the bulk semiconductor 111or carrier blocking layer 114, but can additionally or alternativelyhave any other suitable arrangement. The reduced-doping layer 112 canhave a thickness in the range 100 nm-100 μm (e.g., 1-10 μm, 100-1000 nm,300-3000 nm, 500-1500 nm, etc.), less than 100 nm (e.g., 10-30 nm, 25-65nm, 6-100 nm, less than 10 nm, etc.), greater than 100 μm, and/or anyother suitable thickness. The layer thickness is can be comparable tothe carrier (e.g., electron and/or hole) diffusion length of the layer,substantially greater than the carrier diffusion length, orsubstantially less than the carrier diffusion length. In someembodiments, a lower thickness can be desirable for the reduced-dopinglayer 112, especially at lower doping levels (e.g., to reduce electricalresistance of the layer). However, the reduced-doping layer 112 caninclude any suitable material of any suitable thickness in any suitablearrangement.

The opposite-type layer 113 is preferably doped the opposite type as thebulk semiconductor 111 (e.g., p-type doping in an anode 100 with ann-type bulk semiconductor 111). The opposite-type layer 113 can behighly doped (e.g., equilibrium carrier density greater than a thresholdlevel such as 10¹⁸/cm³, 10¹⁹/cm³, 10²⁰/cm³, 10¹⁷/cm³, 10¹⁶/cm³, etc.),moderately or lightly doped (e.g., equilibrium carrier density less than10¹⁶/cm³, less than 10¹⁴/cm³, less than 10¹²/cm³, in the range10¹⁴/cm³-10¹⁶/cm³, in the range 10¹²/cm³-10¹⁴/cm³, etc.) and/or have anyother suitable doping level.

The opposite-type layer 113 is preferably arranged adjacent to andsuperficial of (e.g., grown epitaxially from) the carrier blocking layer114, reduced-doping layer 112, or bulk semiconductor 111, but canadditionally or alternatively have any other suitable arrangement. Theopposite-type layer 113 is preferably made of the same material as thebulk semiconductor 111 and/or reduced-doping layer 112 (e.g., forming ap-n homojunction with the adjacent semiconductor layer), but canadditionally or alternatively include different semiconductormaterial(s) (e.g., forming a p-n heterojunction with the adjacentsemiconductor layer). For example, a p-type Si opposite-type layer 113can form a p-n junction with an adjacent n-type Si bulk semiconductor111.

The opposite-type layer 113 can have a thickness in the range 100 nm-100μm (e.g., 1-10 μm, 100-1000 nm, 300-3000 nm, 500-1500 nm, etc.), lessthan 100 nm (e.g., 10-30 nm, 25-65 nm, 60-100 nm, less than 10 nm,etc.), greater than 100 μm, and/or any other suitable thickness. Thelayer thickness can be comparable to the carrier (e.g., electron and/orhole) diffusion length of the layer, substantially greater than thecarrier diffusion length, or substantially less than the carrierdiffusion length. In some embodiments, a lower thickness can bedesirable for the reduced-doping layer 112, especially at lower dopinglevels (e.g., to reduce electrical resistance of the layer). However,the opposite-type layer 113 can include any suitable material of anysuitable thickness in any suitable arrangement.

The carrier blocking layer 114 can function to block (e.g., prevent,impede, reduce, etc.) charge carrier transmission (e.g., through thecarrier blocking layer 114 along the superficial-deep axis). The carrierblocking layer 114, cooperatively with one or more adjacentsemiconductor layers 110 and/or other layers, can form a junction thatdefines a significant energy barrier (e.g., substantially greater thank_(B)T at a typical anode operation temperature, such as 250-350° C.) inone band edge (e.g., valence band edge, conduction band edge), andpreferably defines no more than a minimal energy barrier in the otherband edge. The carrier blocking layer 114 is preferably a hole blockinglayer (e.g., defining a large valence band offset and a minimalconduction band offset), but can additionally or alternatively be anelectron blocking layer and/or any other suitable carrier blockinglayer.

The carrier blocking layer 114 is preferably highly doped (e.g.,equilibrium carrier density greater than a threshold level such as10¹⁸/cm³, 10¹⁹/cm³, 10²⁰/cm³, 10¹⁷/cm³, 10¹⁶/cm³, etc.), but canadditionally or alternatively include lower doping (e.g., equilibriumcarrier density less than 10¹⁶/cm³, less than 10¹⁴/cm³, less than10¹²/cm³, in the range 10¹⁴/cm³-10¹⁶/cm³, in the range10¹²/cm³-10¹⁴/cm³, etc.) and/or any other suitable doping level.

The carrier blocking layer 114 is preferably arranged adjacent to andsuperficial of (e.g., grown epitaxially from) the bulk semiconductor111, but can additionally or alternatively have any other suitablearrangement. The carrier blocking layer 114 is preferably made of adifferent material (or materials) than the bulk semiconductor 111,reduced-doping layer 112, and/or other adjacent semiconductor layers(e.g., forming a heterojunction with the adjacent semiconductor layer),but can additionally or alternatively include the same semiconductormaterial.

Additionally or alternatively, the anode can include a carrier blockinglayer 114 arranged between the bulk semiconductor 111 and the electricalcontact 160 (e.g., adjacent to the bulk semiconductor 111 and/or anyother suitable semiconductor layer, proximal the second side of theanode). In this arrangement, the carrier blocking layer 114 can functionto impose a back surface field (e.g., preventing charge carriers of onetype, preferably holes, from reaching the electrical contact 160). Thisback surface field carrier blocking layer is preferably made of the samematerial as the bulk semiconductor 111 and/or other adjacentsemiconductor layers, but can additionally or alternatively include anyother suitable materials. The back surface field carrier blocking layerpreferably exhibits higher doping than the bulk semiconductor. Forexample, the back surface field carrier blocking layer can be formed byimplanting additional dopants (e.g., n-type dopants) into the bulksemiconductor 111 along its rear surface (e.g., proximal the second sideof the anode).

The carrier blocking layer 114 can have a thickness in the range 1-200nm (e.g., 5-50 nm), 200 nm-1 μm, less than 1 nm, greater than 1 μm,and/or any other suitable thickness. The carrier blocking layer 114 ispreferably sufficiently thick to block carrier tunneling through theenergy barrier. For example, the carrier blocking layer 114 thicknesscan be greater than a threshold thickness (e.g., 1 nm, 3 nm, 5 nm, 10nm, etc.). However, the carrier blocking layer 114 can include anysuitable material of any suitable thickness in any suitable arrangement.

In a first variation, the semiconductor layers 110 include ahighly-doped n-type bulk semiconductor 111 and a reduced-doping layer112, which cooperatively define an n-n⁺ homojunction. In a secondvariation, the semiconductor layers 110 include an n-type bulksemiconductor 111, a minimally-doped reduced-doping layer 112 (e.g.,having only unintentional doping), and an opposite-type layer 113, whichcooperatively define a p-i-n homojunction. In a third variation, thesemiconductor layers 110 include, in order of decreasing depth: ahighly-doped n-type GaAs bulk semiconductor 111, an n-type carrierblocking layer 114 of a III-V semiconductor other than GaAs (preferablygallium indium phosphide, but alternatively aluminum gallium arsenide,aluminum gallium indium phosphide, or any other suitable III-Vsemiconductor), an n-type GaAs layer with doping substantially equal tothe bulk semiconductor 111, and an n-type GaAs reduced-doping layer 112.However, the semiconductor layers 110 can additionally or alternativelyinclude any other suitable layers in any suitable arrangement.

1.1.2 Supplemental Anode Layers.

In addition to (or in place of) the semiconductor layer(s) 110, theanode can optionally include one or more supplemental layers 105, suchas layers that function as one or more of: electronic protection layers120, electronic population control layers 125, electron capture layers130, optical tuning layers 135, chemical protection layers 140, workfunction tuning layers 150, and/or any other suitable layers. The anodecan include a single supplemental layer (e.g., which provides one ormore than one of the functionalities described herein regarding thesupplemental layers), multiple supplemental layers, or no supplementallayer.

The electronic protection layer 120 can function to passivate (e.g.,minimize electronic traps at and/or near) one or more semiconductorsurfaces and/or interfaces (e.g., most superficial semiconductorinterface of the anode). However, the electronic protection layer 120can additionally or alternatively function to include moderate and/orhigh densities of electronic defects (e.g., to achieve Fermi-levelpinning at a desired energy level) in any suitable locations, and/or tocontrol any other suitable interface aspects. The electronic protectionlayer 120 is preferably adjacent to, and more preferably superficial of,the semiconductor layer(s) 110 that it protects.

The electronic protection layer 120 preferably allows efficient electrontransport through itself (e.g., from superficial toward deep). Forexample, the electronic protection layer 120 can be sufficiently thin toenable efficient tunneling through it (e.g., less than a thresholdthickness, such as 1, 3, or 5 nm), and/or can have a conduction bandedge that enables electrons to travel through it (e.g., substantiallyaligned with the conduction band edges of adjacent layers; conductionband edges of the electronic protection layer 120 and adjacent layersstaggered or sloped, such as with decreasing energy at increasing depthin the anode; etc.). The electronic protection layer 120 canadditionally or alternatively block (e.g., prevent, reduce, etc.) holetransport through itself (e.g., can function as a carrier blocking layer114). For example, the electronic protection layer valence band edge candefine a large offset from one or more of the adjacent layers (e.g.,presenting a large energetic barrier to holes entering and/or exitingthe layer).

The electronic protection layer 120 can have a thickness less than athreshold thickness (e.g., 10 nm, 100 nm, 1 μm, etc.). For example, thethickness can be 1-10 nm, 10-25 nm, 25-100 nm, or less than 1 nm.However, the electronic protection layer 120 can alternatively have athickness greater than 1 μm, or have any other suitable thickness.

The electronic protection layer 120 preferably includes (e.g., ispreferably made of) an insulator or semiconductor (e.g., having a widerbandgap than one or more of the materials in the semiconductor layers110). In a first variation, the electronic protection layer 120 includesa semiconductor-based compound. The compound can include the samematerial as the semiconductor layers 110, and/or can include one or moreother semiconductors. In a first example of this variation, theelectronic protection layer 120 includes a semiconductor-oxide compound(e.g., a native oxide or thermal oxide, such as silicon oxide grown froman underlying silicon layer; residual portions of a native oxide, suchas after partial removal of the oxide under treatment such as thermaland/or chemical treatment; etc.). In a specific example, the electronicprotection layer 120 includes (e.g., substantially is) a silicon oxidelayer with thickness less than 10 nm (e.g., 0.05-0.5 nm, 0.25-1 nm,0.5-3 nm, 2-5 nm, 3.5-7 nm, 5-10 nm, etc.). In a second example, theelectronic protection layer 120 includes a semiconductor-nitride and/or-oxynitride compound (e.g., silicon nitride).

In a second variation, the electronic protection layer 120 includes ametal compound. The compound is preferably a transition metal (e.g.,titanium, tantalum, molybdenum, hafnium, lanthanum, etc.) compound, butcan additionally or alternatively include any other suitable metalelements. In a first example of this variation, the compound is an oxide(e.g., titanium oxide, tantalum oxide, molybdenum oxide, etc.), nitride,or oxynitride. In a second example, the compound is a silicide (e.g.,nickel silicide).

In a third variation, the electronic protection layer 120 includes awide bandgap semiconductor (e.g., GaN). In a fourth variation, theelectronic protection layer 120 includes a 2D material (e.g., graphene,BN, MoS₂, MoSe₂, etc.). However, the electronic protection layer 120 canadditionally or alternatively include any other suitable materials.

The electronic population control layer 125 can function to affect theelectron and/or hole population (e.g., concentration, energy levels,etc.) within the semiconductor layers 110 (e.g., at and/or near thesuperficial side of the semiconductor layers). The layer 125 preferablyfunctions to increase the built-in voltage of the anode (e.g., therebyenhancing the potential photovoltage effect, which can result in greaterwork function reduction), but can additionally or alternatively performany other suitable functions.

In a first embodiment, the electronic population control layer 125 isalso an electronic protection layer 120, such as a layer arranged near(e.g., just superficial of) the superficial side of the semiconductorlayers. In a second embodiment, the electronic population control layer125 is arranged near (e.g., just superficial of) the superficial side ofthe electronic protection layer 120 (e.g., wherein the electronicprotection layer 120 is arranged adjacent the superficial side of thesemiconductor layers 110). However, the electronic population controllayer 125 can alternatively have any other suitable arrangement.

The electronic population control layer 125 preferably causes the Fermilevel to lie (e.g., due to Fermi level pinning) near the semiconductorvalence band edge (e.g., within a threshold energetic distance of thevalence band, such as 10, 20, 50, 75, 100, 150, 200, 250, 300, 400, 500,5-25, 20-50, 40-100, 75-200, 150-350, or 300-500 meV; within afractional amount of the semiconductor bandgap, such as 0.5%, 1%, 2%,3%, 5%, 7.5%, 10%, 15%, 20%, 25%, 30%, 40%, 0.5-2.5%, 2-5%, 4-10%,7.5-20%, or 15-40%; etc.). However, the layer 125 can additionally oralternatively cause the Fermi level to lie near (e.g., within thethreshold energetic distance or fractional amount of the semiconductor)the conduction band edge, the mid-gap energy level, and/or any othersuitable energy level of the semiconductor and/or any other suitablematerial of the anode.

The layer 125 can affect the electron population via electronic defectsand/or charge neutrality level at a desired energy level (e.g., energylevel of desired Fermi level pinning, such as at or near thesemiconductor valence band edge). The electronic defects are preferablyarranged near the superficial side of the layer 125 (e.g., therebyreducing likelihood of charge carriers within the semiconductorrecombining at the defects), but can additionally or alternatively bearranged near the deep side (e.g., at the interface with thesemiconductor layers) and/or in the bulk of the layer 125. The chargeneutrality level is preferably associated with a low Schottky pinningparameter, but can alternatively be exhibited by a material with anysuitable Schottky pinning parameter.

The electronic population control layer 125 can have a thickness lessthan a threshold thickness (e.g., 10 nm, 100 nm, 1 μm, etc.). Forexample, the thickness can be 1-10 nm, 10-25 nm, 25-100 nm, or less than1 nm. However, the electronic protection layer 120 can alternativelyhave a thickness greater than 1 μm, or have any other suitablethickness.

The electronic population control layer 125 preferably includes (e.g.,is made of) a metal and/or metal compound. The metal is preferably atransition metal (e.g., titanium, tantalum, molybdenum, hafnium,lanthanum, etc.), but can additionally or alternatively include anyother suitable metal elements. In a first example of this variation, thecompound is an oxide (e.g., titanium oxide, tantalum oxide, molybdenumoxide, etc.), nitride, or oxynitride. In a second example, the compoundis a silicide (e.g., nickel silicide). In a third example, the materialis metallic (e.g., titanium metal, molybdenum metal, tungsten metal,iridium metal, etc.). However, the layer 125 can additionally oralternatively include any other suitable materials.

In one example, the layer 125 (e.g., a layer of titanium oxide) isdeposited (e.g., onto the semiconductor layers 110) by a thin filmgrowth technique such as atomic layer deposition (e.g., thermal ALD,plasma ALD, etc.). Such a deposition technique may result in formationof an additional oxide layer between the semiconductor layers 110 andthe electronic population control layer 125 (e.g., when thesemiconductor layers are silicon, a thin silicon oxide layer may formbetween the silicon and the electronic population control layer 125). Inthis example, the layer 125 is preferably deposited at a relatively highdeposition temperature (e.g., greater than or equal to a thresholdtemperature, such as 1000, 900, 800, 700, 600, 500, 400, 300, 250, 200,150, 150-350, 300-500, 400-600, 500-700, 650-850, or 800-1000° C.),which can promote formation of desirable electronic defects. In aspecific example, a layer of titanium oxide is deposited by thermal ALDat a temperature of 200-300° C. (e.g., approximately 250° C., such as225-275, 235-265, or 245-255° C.), preferably using water and one ormore titanium precursor species such as tetrakis(dimethylamido)titanium(TDMAT) and/or titanium isopropoxide (TIP). However, the layer 125 canadditionally or alternatively be deposited at a moderate or lowtemperature (e.g., less than or equal to the threshold temperature)and/or under any other suitable conditions.

The electron capture layer 130 can function to capture electrons (e.g.,electrons thermionically emitted from the cathode) and/or to enablecaptured electron transport deeper into the anode (e.g., to thesemiconductor layers 110, such as to the bulk semiconductor 111). Theelectron capture layer 130 is preferably configured (e.g., the materialsand/or configurations are selected) to minimize evaporation,interdiffusion, and/or deleterious interaction (e.g., at a typicalelevated anode operation temperature, such as 150-250° C., 250-350° C.,450-550° C., 550-650° C., 100-1000° C., and/or any other suitable anodetemperature) with the other layers of the anode (e.g., work functiontuning layer 150).

The electron capture layer 130 can have a thickness less than athreshold thickness (e.g., 10 nm, 100 nm, etc.). For example, thethickness can be 1-10 nm, 10-25 nm, 25-100 nm, or less than 1 nm.However, the electronic protection layer 120 can alternatively have athickness greater than 100 nm, or have any other suitable thickness.

The electron capture layer 130 preferably includes (e.g., is preferablymade of) one or more materials that provide a high electron density ofstates (e.g., in the conduction band and/or close to the vacuum level,such as within 0.05, 0.1, 0.2, 0.3, or 0.5 eV of the vacuum level)and/or a high effective electron mass (e.g., greater than or equal to athreshold multiple of the free electron mass, such as 0.75, 1, 1.1, 1.2,1.3, 1.4, 1.5, 1.75, 2, 2.5, 0.7-1, 0.9-1.1, 1-1.3, 1.2-1.5, 1.4-1.6,1.5-1.8, 1.75-2.25, 2-3, etc.), which can function to increase electroncapture in the layer.

In a first variation, the electron capture layer 130 includes one ormore metals. In a first example of this variation, the layer includes atransition metal (e.g., Ni, W, Mo, etc.). In a second example, the layerincludes an alkali or alkaline earth metal (e.g., Cs, Ba, Sr, etc.),which can additionally or alternatively function as a work functiontuning layer 150 and/or as a reservoir for a work function tuning layer150 (e.g., replenishing depleted material from the work function tuninglayer 150).

In a second variation, the electron capture layer 130 includes a metalcompound. The compound is preferably a transition metal (e.g., titanium,tantalum, molybdenum, hafnium, lanthanum, etc.) compound, but canadditionally or alternatively include any other suitable metal elements.In a first example of this variation, the compound is an oxide (e.g.,titanium oxide, tantalum oxide, molybdenum oxide, etc.), nitride (e.g.,titanium nitride, tantalum nitride, etc.), or oxynitride. In a secondexample, the compound is a silicide (e.g., nickel silicide).

In a third variation, the electron capture layer 130 includes a 2Dmaterial (e.g., graphene, BN, MoS₂, MoSe₂, etc.). In a fourth variation,the electron capture layer 130 includes a boron compound (e.g.,hexaboride such as LaB₆, CeB₆, BaB₆, etc.).

The electron capture layer 130 can include any suitable combinations(e.g., alloys, mixtures, etc.) of the above materials and/or any othersuitable materials. In a first specific example, the electron capturelayer 130 includes a mixture of titanium nitride and tungsten. In asecond specific example, the layer 130 includes a transition metal oxidewith LaB₆ inclusions. However, the electronic protection layer 120 canadditionally or alternatively include any other suitable materials withany other suitable structure.

The optical tuning layer 135 can function to tune the anode'sinteraction with incident light (e.g., radiation emitted by the cathode,enclosure, spacers, and/or other elements of the TEC, such as thermalradiation; light from external sources, such as sunlight; etc.). Forexample, the optical tuning layer 135 can function to increase or reducethe amount of light, preferably above-gap light (e.g., photons withenergy about the semiconductor bandgap), absorbed by the bulksemiconductor and/or other semiconductor layers (e.g., as compared withabsorption in an otherwise substantially identical anode in which theoptical tuning layer is different or absent). The optical tuning layer135 can additionally or alternatively function to reduce the amount oflight absorbed by the anode outside the semiconductor layers, reduce thesub-gap light (e.g., photons with energy less than the semiconductorbandgap) absorbed by the anode (e.g., including absorption within thesemiconductor layers, such as free-carrier absorption), function tocontrol which regions within the semiconductor light is absorbed in(e.g., promoting absorption near the first side of the anode, reducingabsorption near the second side of the anode and/or deep within the bulksemiconductor, etc.), and/or function to tune optical properties of theanode (and/or other elements of the system) in any other suitablemanner.

The optical tuning layer 135 is preferably integrated with anothersupplemental layer 105 (e.g., wherein the other supplemental layeradditionally functions as an optical tuning layer). For example, theelectronic protection layer 120, electronic population control layer125, electron capture layer 130, and/or chemical protection layer 140can function as an optical tuning layer 135. However, the supplementallayers 105 can additionally or alternatively include a separate opticaltuning layer 135 (e.g., layer included solely or primarily for opticaltuning functionality), the optical tuning layer 135 can be integratedwith a semiconductor layer (e.g., wherein a semiconductor layer, such asthe carrier blocking layer, additionally functions as an optical tuninglayer), and/or the optical tuning can be achieved in any other suitablemanner.

The optical tuning layer 135 can have a thickness less than a thresholdthickness (e.g., 10 nm, 100 nm, etc.). For example, the thickness can be1-10 nm, 10-25 nm, 25-100 nm, or less than 1 nm. However, the electronicprotection layer 120 can alternatively have a thickness greater than 100nm, or have any other suitable thickness. The optical tuning layer 135(and/or any other supplemental layers 105) can have a refractive indexgreater than 1 (e.g., 1.1, 1.2, 1.3, 1.4, 1.5, 1.75, 2, 2.25, 2.5, 3,1-1.3, 1.2-1.5, 1.5-2, 2-3, greater than 3, etc.), substantially equalto 1, between 0 and 1, less than 0, and/or any other suitable refractiveindex. The optical tuning layer 135 can include: metals, metal compounds(e.g., oxides, nitrides, etc.), semiconductors, semiconductor compounds(e.g., oxides, nitrides, etc.), and/or any other suitable materials.

In a first embodiment, the optical tuning layer 135 functions toincrease above-gap light absorption in the semiconductor layers (e.g.,to enhance the photovoltage effect). In a first example of thisembodiment, the layer 135 functions as an anti-reflective coating. Inthis example, the layer 135 preferably has an intermediate refractiveindex (e.g., with a value between the indices of the semiconductorlayers and the gap adjacent the anode first side), such as a valuegreater than 1 but less than the semiconductor refractive index. In asecond example, the layer 135 includes a textured topography (e.g.,pyramidal or inverse pyramidal, domed, etc.), which can function topromote light scattering (e.g., increasing photon path length within thesemiconductor material, thereby increasing absorption). In a thirdexample, the layer 135 functions to enhance above-gap light intensity ina region of desired absorption (e.g., near a side of the semiconductorlayers closest to the anode first side) and/or decrease light intensityin a region of undesired absorption (e.g., near the opposing side of thesemiconductor layers, where above-gap absorption can result inphotovoltage effects that reduce the device output voltage), such as viaconstructive and/or destructive optical interference (e.g., achieved incooperation with one or more other layers of the anode, such as areflective electrical contact 160 opposing the layer 135 across thesemiconductor layers 110). However, the layer 135 can additionally oralternatively increase above-gap absorption in any other suitablemanner.

The optical tuning layer 135 can additionally or alternatively functionto reduce light absorption in the anode (e.g., to promote retransmissionof the light to the cathode, thereby reducing thermal losses from thecathode and/or heating of the anode), optionally including reduction ofabove-gap light absorption in the semiconductor layers. For example, thelayer 135 can be an optical reflector (e.g., can have a high refractiveindex, such as a refractive index greater than the semiconductorrefractive index). In a first example, the layer 135 includes a metal ormetal compound that functions as a broad-spectrum reflector. In a secondexample, the layer 135 includes (e.g., in the semiconductor layers 110,such as integrated with and/or near the carrier blocking layer 114) oneor more wider-bandgap (as compared with the bulk semiconductor 111)semiconductor layers, preferably moderately- or highly-doped layers.Such wider-bandgap layers can be reflective to below-gap photons (e.g.,IR photons) but substantially transparent to above-gap photons (e.g.,visible light photons). However, the anode 100 can additionally oralternatively include any suitable optical tuning layer(s) 135 in anysuitable configuration.

The chemical protection layer 140 can function to protect materials inone or more other layers (e.g., deeper than the chemical protectionlayer 140) from chemical and/or electrochemical degradation. Forexample, the chemical protection layer 140 can function to protect thesemiconductor layers 110 from chemical degradation due to interactionwith a work function tuning material (e.g., material from the workfunction tuning layer 150) and/or other potentially reactive substancesin the environment of the anode (e.g., oxygen), such as degradation atelevated temperature (e.g., a typical anode operation temperature, suchas 150-250° C., 250-350° C., 450-550° C., 550-650° C., 100-1000° C.,and/or any other suitable anode temperature). In a first example, thelayer 140 prevents passage (e.g., diffusion) of the reactive and/ordegradative species through the layer 140. In a second example, thelayer 140 captures the reactive and/or degradative species (e.g., bychemically reacting with them, such as titanium metal and/or oxygen-poortitanium oxide scavenging oxygen, thereby preventing it from oxidizingthe semiconductor layers 110). However, the layer 140 can additionallyor alternatively function in any other suitable manner.

The chemical protection layer 140 is preferably present in embodimentsof the anode 100 that include incompatible (e.g., at elevatedtemperature, such as a typical anode operation temperature; at ambienttemperature; etc.) semiconductor and work function tuning materials(e.g., GaAs and Cs). However, any suitable chemical protection layer 140can be used in any suitable embodiment of the anode 100. The chemicalprotection layer 140 is preferably arranged between one or more layers,but can additionally or alternatively be arranged along the edge of thesystem 10, a face of the system 10, or be otherwise arranged.

The chemical protection layer 140 is preferably conformal to theunderlying layer (e.g., adjacent deeper layer), and preferably includesno or minimal structural defects (e.g., pinholes, voids, cracks, etc.),which can enable the layer 140 to function as an effective chemicalbarrier (e.g., preventing Cs transport through the layer). The layer 140preferably withstands repeated (e.g., 1-5 cycles per day for 5-50 years)thermal cycling (e.g., between an ambient temperature such as 20° C. andan anode operation temperature such as 150-250° C., 250-350° C.,450-550° C., 550-650° C., 100-1000° C., and/or any other suitable anodetemperature) with minimal damage (e.g., minimal development ofstructural defects). However, the chemical protection layer 140 can haveany suitable structural and/or mechanical properties.

The chemical protection layer 140 can have a thickness in the range0.1-10 nm, greater than 10 nm, or less than 0.1 nm. In some embodiments,a thinner chemical protection layer 140 can reduce layer interactionswith electrons (e.g., enabling efficient electron transmission). Inother embodiments, a thicker chemical protection layer 140 can provide amore effective chemical barrier (e.g., minimizing damage to underlyinglayers from reactive work function tuning materials). However, thechemical protection layer 140 can have any suitable thickness.

The chemical protection layer 140 can include (e.g., be made of)insulators (e.g., similar materials as described above regarding theelectronic protection layer 120, different materials from the electronicprotection layer 120), semiconductors (e.g., the same materials as inthe semiconductor layers 110, different semiconductor materials), metals(e.g., transition metals, such as Ni, Mo, W, etc.), and/or any othersuitable materials. However, the chemical protection layer 140 caninclude any other suitable materials with any other suitable structure.

The work function tuning layer 150 can function to alter the anode workfunction (e.g., in addition to and/or in place of photovoltage-basedwork function changes), preferably to reduce the work function (e.g.,wherein the work function tuning layer 150 is a work function reductionlayer) but alternatively to increase the work function. The workfunction tuning layer 150 is preferably very thin (e.g., monolayer, fewmonolayers, sub-monolayer, etc.), but can alternatively be thicker. Forexample, the layer 150 can be less than 1 nm, less than 10 nm, greaterthan 10 nm, and/or have any other suitable thickness. The work functiontuning layer 150 is preferably arranged at (and/or near) the superficialsurface of the anode, but can additionally or alternatively have anyother suitable arrangement.

In a first embodiment, the work function tuning layer 150 defines one ormore substantially sharp interfaces (e.g., wherein an interfacial regionbetween the work function tuning layer 150 and an adjacent layer islimited to substantially as thin as practical, such as described above;wherein the work function tuning layer 150 does not substantiallypenetrate the adjacent layer, such as by diffusion into the adjacentlayer). In a second embodiment, the work function tuning layer 150 formsa substantial interfacial region (e.g., as described above) with one ormore adjacent layers. In one example, cesium (and optionally oxygen) isdeposited onto a supplemental layer 105 (e.g., titanium oxidesupplemental layer) to form a work function tuning layer 150. In thisexample, some of the cesium (and/or oxygen) may penetrate thesupplemental layer 105, creating an interfacial region (e.g., gradientof cesium concentration within the titanium oxide). However, the workfunction tuning layer 150 can additionally or alternatively define anyother suitable interfaces.

The work function tuning (e.g., reduction) effects achieved by the workfunction tuning layer 150 are preferably independent (or substantiallyindependent) of any photovoltage-based work function tuning effects. Forexample, the total change in work function between an unilluminatedanode with no work function tuning layer and an illuminated anode with awork function tuning layer is preferably substantially equal (e.g.,within a threshold value, such as 10, 50, 100, 150, 200, or 300 meV;within a threshold fraction of the work function or change in workfunction, such as 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, etc.) to the sumof the change attributable to the work function tuning layer alone(e.g., for an unilluminated anode) and the photovoltage-based effectalone (e.g., for an anode with no work function tuning layer). However,the work function tuning effects can alternatively not be substantiallyindependent, and/or can have any other suitable relationship.

The work function tuning layer 150 can include (e.g., be made of) anysuitable work function tuning materials. In a first variation, the layer150 includes one or more metals. In a first example of this variation,the metals include alkali or alkaline earth metals (e.g., Cs, Ba, Sr,Ca, etc.). In a second example, the metals include one or more rareearth elements (e.g., La, Ce, etc.). In a second variation, the layer150 includes a 2D material (e.g., graphene, BN, MoS₂, MoSe₂, etc.).

In a third variation, the work function tuning layer 150 includes one ormore metal compounds (e.g., compounds containing oxygen, fluorine,and/or boron). In a first example of this variation, the compoundsinclude oxides of alkali or alkaline earth metals (e.g., Cs—O). In asecond example, the compounds include diboride and/or hexaboridecompounds (e.g., LaB₆, CeB₆, BaB₆, etc.). The diboride and/or hexaboridecompounds can be stoichiometric, boron-rich, and/or boron-poor. Forexample, the layer 150 can include an LaB₆—BaB₆ superlattice, and/or thelayer 150 can include inclusions of La, Zr, V, B, and/or compoundsthereof.

In one embodiment, the work function tuning layer 150 can include a thinCs or Cs—O coating formed from a Cs vapor environment adjacent to theanode surface (e.g., first surface). In one example of this embodiment,the supplemental layers include (e.g., at the superficial surface of theanode, superficial to all other layers except the work function tuninglayer, etc.) one or more materials on which Cs (and/or other workfunction reducing materials) exhibits Stranski-Krastanov growth (e.g.,layer-plus-island growth, such as growth in which a uniform layer ofpolarized Cs grows on the surface, followed by and/or along with islandsof greater Cs growth), such as titanium oxide and/or other suitableoxides. Such Stranski-Krastanov Cs growth can function to help avoidlarge-scale depolarization of the Cs layer (e.g., in the presence ofexcess Cs, such as significantly more Cs than is needed to form thepolarized layer), thereby allowing for a large window of Cs conditionsin which the majority of the anode surface maintains a significantlydecreased work function (e.g., wherein only regions associated with theadditional islands of Cs exhibit substantial hampering of the workfunction reduction effects of Cs). However, the material(s) of the workfunction tuning layer (e.g., Cs) can additionally or alternativelyexhibit island growth, layer growth, and/or any other suitable growthmechanism (e.g., the layer superficial to all other layers except thework function tuning layer can support such growth of the material).However, the work function tuning layer 150 can additionally oralternatively include any suitable material.

The supplemental layers can optionally include (e.g., in addition toand/or in place of a superficial coating such as Cs) a layer of highwork function material that functions as a work function tuning layer150. The layer is preferably very thin (e.g., monolayer, few monolayers,sub-monolayer, 0.1 nm, 0.5 nm, 1 nm, 2 nm, 5 nm, 0.1-0.5 nm, 0.5-2 nm,2-5 nm, etc.), but can alternatively be thicker (e.g., 5-10 nm, 10-20nm, 20-50 nm, greater than 50 nm, etc.). The layer can include (e.g., bemade of) a metal oxide, such as molybdenum oxide, manganese oxide,tungsten oxide, an oxide of manganese and one or more other metals; ametal (e.g., Ir, Au, Rh, Os, Re, Ru, Ti, Mo, W, Cr, etc.); and/or anyother suitable high work function materials (e.g., material with workfunction in excess of a threshold value, such as 2, 3, 3.5, 4, 4.25,4.5, 4.6, 4.7, 4.8, 4.9, 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9,6, 3.5-4.5, 4.5-5, 5-5.5, 5.5-6, or 6-7 eV, etc.). The layer ispreferably arranged near (e.g., adjacent to) the semiconductor layers110 (e.g., just superficial of the semiconductor layers), such asbetween the semiconductor layers and another supplemental layer (e.g.,another oxide layer, such as titanium oxide). In a specific example, thesemiconductor layers include (e.g., are made substantially of) silicon,and the supplemental layers include a thin layer of manganese oxideand/or molybdenum oxide arranged between the silicon and a titaniumoxide layer. However, the work function tuning layer 150 canadditionally or alternatively include any other suitable materials.

In one embodiment, the anode 100 includes a single supplemental layer105 that performs a plurality of the above functions. For example, thesupplemental layer 105 can be a metal oxide (e.g., titanium oxide). Themetal oxide can function, for example, as an electronic protection layer120, an electronic population control layer 125, an electron capturelayer 130, and/or a chemical protection layer 140. In this embodiment,the anode 100 can optionally include additional supplemental layers 105(e.g., to perform additional functions, to supplement the functions ofthe first supplemental layer, etc.). For example, the metal oxide layercan be arranged just superficial of the semiconductor layers, and a workfunction tuning layer 150 (e.g., Cs coating) can be arranged justsuperficial of the metal oxide layer. However, the supplemental layers105 can additionally or alternatively have any other suitablearrangement, include any other suitable materials, and/or provide anyother suitable functions.

1.1.3 Electrical Contact.

The electrical contact 160 can function to extract electrons from theanode 100 (e.g., to drive an electrical load). The electrical contact160 is preferably arranged on the second side of the anode, but canadditionally or alternatively be arranged on the first side (e.g.,laterally patterned such as a ring contact, thin continuous layer,etc.), between the sides (e.g., within the anode 100), and/or have anyother suitable arrangement. The electrical contact 160 is preferablyelectrically connected to one or more of the semiconductor layers 110,more preferably forming a good electrical contact (e.g., ohmic contact;Schottky contact with negligible or minimal energy barrier relative tothe anode operating temperature, such as 0.01, 0.05, 0.1, 0.2, 0.5,0.75, 1, 1.5, 2, 2.5, 3, 4, 5, 10, 0.01-0.1, 0.1-0.5, 0.5-1.5, 1.5-3,3-5, 5-10, 10-20, or 20-50 times k_(B)T at the anode operatingtemperature; etc.) to the semiconductor.

The electrical contact 160 optionally includes an adhesion layer, whichcan function to enhance electrical contact adhesion to thesemiconductor. The adhesion layer is preferably arranged adjacent to(e.g., deposited directly onto) the semiconductor layers, but canadditionally or alternatively have any other suitable arrangement. Forexample, the adhesion layer can include a coating, preferably a thin(e.g., 1-10 nm, 0.1-1 nm, 10-30 nm, etc.) coating but alternatively acoating of any suitable thickness, of titanium (e.g., which canadditionally or alternatively function to scavenge oxygen, therebypreventing oxidation of the semiconductor) and/or other metals thatadhere well to the semiconductor (e.g., deposited onto thesemiconductor).

The electrical contact 160 preferably includes a diffusion barrier,which can function to prevent metal species (e.g., of the electricalcontact) from diffusing into and/or reacting with the semiconductorlayers. Thus, the diffusion barrier is preferably arranged between thesemiconductor layers and the majority of the electrical contact (e.g.,wherein only the adhesion layer separates the diffusion barrier from thesemiconductor layers). In a first example, the diffusion barrierincludes one or more metals that do not readily react with thesemiconductor (e.g., for a silicon bulk semiconductor, do not formsilicides) at the anode operating temperature, such as molybdenum and/ortungsten. In this example, the diffusion barrier is preferably tens ofnm thick (e.g., 10-100 nm, such as 10-25, 20-50, 40-70, 50-60, 60-80, or80-100 nm), but can alternatively be thicker than 100 nm or thinner than10 nm. In a second example, the diffusion barrier includes one or morecompounds (e.g., nitrides, oxides, etc.) of metals and/or other species(e.g., titanium nitride, tantalum nitride, etc.), which can function toprevent metal diffusion. In this example, the diffusion barrier ispreferably sufficiently thin to allow efficient electron transportacross the barrier (e.g., 0-1, 1-2, 2-5, 5-10, 10-20 nm, etc.), but canalternatively have any other suitable thickness. However, the diffusionbarrier can additionally or alternatively include any suitable materialsin any suitable configuration.

The electrical contact 160 preferably includes a thick (e.g., greaterthan a threshold thickness, such as 100 or 1000 nm), low-resistancemetal structure, which can efficiently conduct (e.g., laterally,out-of-plane, etc.) device currents in excess of 1 or 10 A/cm². Thisstructure can include a continuous layer, bus bars and/or wires, and/orany other suitable elements. The structure can be formed on and/orattached to the anode by electrodeposition, wafer bonding, and/or anyother suitable fabrication technique.

The electrical contact 160 is preferably optically reflective (e.g., inembodiments in which the electrical contact 160 is arranged on thesecond side, whereby light reaching the electrical contact 160 can bereflected back through the anode 100 and/or to the cathode 200), but canadditionally or alternatively absorb and/or transmit incident light. Theelectrical contact 160 (and/or other elements near the anode secondside, such as proximal the second side relative to the semiconductorlayers) can additionally or alternatively include a textured topography(e.g., pyramidal or inverse pyramidal, domed, etc.), which can functionto promote light scattering (e.g., increasing photon path length withinthe semiconductor material, thereby increasing absorption).

The electrical contact 160 preferably includes (e.g., is preferably madeof) one or more metals, metal compounds (e.g., silicides, oxides, etc.),and/or layered stacks thereof (e.g., Pt deposited after Ti and/or Ni).In a first variation, in which the bulk semiconductor 111 is silicon,the electrical contact 160 makes ohmic contact to silicon (e.g., Al,Al—Si, TiSi₂, TiN, W, MoSi₂, PtSi, CoSi₂, WSi₂, etc.). In a secondvariation, in which the bulk semiconductor 111 is GaAs, the electricalcontact 160 makes ohmic contact to GaAs (e.g., AuGe, PdGe, PdSi,Ti/Pt/Au, etc.). However, the electrical contact 160 can additionally oralternatively include any other suitable materials.

1.1.4 Examples.

In one embodiment, the anode 100 includes a bulk semiconductor 111,preferably includes an electrical contact 160 on the back side of thebulk semiconductor 111 (e.g., proximal the anode second side),preferably includes a work function tuning layer 150 (e.g., Cs or Cs—Ocoating), such as at the anode first side, and optionally includes oneor more additional layers (e.g., intermediary layers arranged betweenthe bulk semiconductor 111 and the work function tuning layer 150; inthe absence of a work function tuning layer, layers arranged superficialof the bulk semiconductor 111; etc.). For example, the bulksemiconductor can be a bulk n-type silicon wafer, such as a 50-250 μmthick wafer (e.g., 100 μm, 200 μm, etc.) and/or a wafer with anequilibrium carrier concentration of 10¹⁶-10¹⁸/cm³ (e.g., 10¹⁶, 2×10¹⁶,5×10¹⁶, 7.5×10¹⁶, 10¹⁷, 2×10¹⁷, etc.).

It can be instructive to compare (measured and/or calculated) banddiagrams of such structures in the dark and under illumination. Thetypical illumination expected during device operation, such as receivedfrom cathode thermal radiation, may fall in the range of 10¹³-10²¹ cm⁻²s⁻¹ above-gap photons, more typically (e.g., for a tungsten ormolybdenum cathode at a temperature of 1300-2000° C. and a siliconanode) 10¹⁵-10²⁰ cm⁻² s⁻¹ (e.g., for a 300° C. silicon anode and a 1500°C. tungsten cathode, typically 10¹⁶-10¹⁸ cm⁻² s⁻¹, such as approximately4×10¹⁷ cm⁻² s⁻¹), but the illumination can additionally or alternativelybe greater than 10²¹ cm⁻² s⁻¹ (e.g., 10²¹-10²⁵ cm⁻² s⁻¹, greater than10²⁵ cm⁻² s⁻¹, etc.), less than 10¹³ cm⁻² s⁻¹ (e.g., 10⁹-10¹³ cm⁻² s⁻¹,less than 10⁹ cm⁻² s⁻¹, etc.), and/or have any other suitable value.However, band diagrams of such structures under elevated illumination,such as 10,000 suns illumination, can also be instructive. These banddiagrams can illustrate the potential and/or realized effects of thephotovoltage-based work function reduction, which may typically scaleapproximately logarithmically with the illumination intensity.

In a first example, there is no additional layer, and there isoptionally a work function tuning layer 150 (e.g., Cs, Cs—O, etc.) atthe anode first side. For a specific example of such an anode, with a100 μm thick n-type silicon wafer doped to an equilibrium carrierconcentration of 10¹⁷/cm³, calculated band diagrams in the dark (dashedlines) and under 10,000 suns illumination (solid lines) are shown inFIG. 5.

In a second example, there is optionally a work function tuning layer150 (e.g., Cs, Cs—O, etc.) at the anode first side, and the anodeincludes a titanium oxide layer, preferably 3-25 nm thick (e.g., 3-10nm, 5-15 nm, 10-25 nm, etc.). The titanium oxide layer is preferablydeposited by atomic layer deposition (e.g., thermal, plasma, etc.), butcan additionally or alternatively be formed in any other suitablemanner. The titanium oxide layer is preferably adjacent the bulksemiconductor, but can alternatively be in any other suitablearrangement. For a specific example of such an anode, with a 10 nm thicktitanium oxide layer adjacent a 100 μm thick n-type silicon wafer dopedto an equilibrium carrier concentration of 10¹⁷/cm³, calculated banddiagrams in the dark (dashed lines) and under 10,000 suns illumination(solid lines) are shown in FIG. 6.

In a third example, there is optionally a work function tuning layer 150(e.g., Cs, Cs—O, etc.) at the anode first side, and the anode includes ap-type silicon layer adjacent the bulk semiconductor (e.g., formed byion implantation into the n-type silicon wafer, such as implantation of4×10¹⁵/cm² boron ions at 15 keV). For a first specific example of suchan anode, with a 100 μm thick n-type silicon wafer doped to anequilibrium carrier concentration of 10¹⁷/cm³, calculated band diagramsin the dark (dashed lines) and under 10,000 suns illumination (solidlines) are shown in FIG. 7. In a second specific example, the anodeoptionally includes one or more of a high work function metal (e.g.,iridium) and a compound with electronic defects near the semiconductorvalence band (e.g., titanium oxide), arranged superficial of thesemiconductor layers (e.g., when a work function tuning layer ispresent, arranged between the semiconductor layers and the work functiontuning layer). When both the metal and the compound are present, themetal is preferably arranged superficial of the compound, but canalternatively be arranged between the semiconductor layers and thecompound.

In a fourth example, the anode includes a device such as described inApplication of Semiconductors to Thermionic Energy Converters by DanielC. Riley, which is hereby incorporated in its entirety by this reference(e.g., as described in Chapter 4, entitled “Surface Photovoltage”),modified to include one or more additional layers such as thosedescribed above (e.g., intermediary layers arranged between the bulksemiconductor 111 and the work function tuning layer 150; in the absenceof a work function tuning layer, layers arranged superficial of the bulksemiconductor 111; etc.). In a specific example, the anode includes adevice such as described in Riley's Chapter 4, but modified to include awork function reduction layer (e.g., Cs, Cs—O, etc.) and a compound(e.g., thin layer of the compound, such as 3-30 nm, 0.3-3 nm, 30-60 nm,etc.) with electronic defects near the semiconductor valence band (e.g.,titanium oxide), wherein the compound is arranged between thesemiconductor and the work function reduction layer.

However, the anode 100 can additionally or alternatively include anyother suitable elements (e.g., layers) in any suitable arrangement.

1.2 Cathode.

The cathode 200 can function to emit electrons (e.g., thermionically).The cathode 200 is preferably substantially planar (e.g., is a flatwafer), but can additionally or alternatively define any suitable shape.The cathode 200 can optionally include surface textures, lateralfeatures, and/or any other suitable features.

The cathode 200 preferably includes (e.g., is preferably made of) one ormore metals (e.g., refractory metals), and can additionally oralternatively include semiconductors, insulators, and/or any othersuitable materials. The cathode 200 can include a work function tuninglayer (e.g., as described above regarding the anode work function tuninglayer 150, otherwise). The cathode 200 preferably has a low workfunction (e.g., less than a threshold value, such as 4 eV, 3.5 eV, 3 eV,2.5 eV, 2 eV, 1.5 eV, 1 eV, 0.5-5 eV, etc.), but can additionally oralternatively have any suitable work function.

The cathode 200 can optionally include elements that function to controlphoton transfer from the cathode to the anode (e.g., working aloneand/or in cooperation with the anode optical tuning layer 145). Forexample, the cathode can include a material (e.g., metal, such astungsten or molybdenum) that exhibits favorable emissivity (e.g., higheremissivity of above-gap photons than below-gap photons), such asincluding a bulk portion of the material (e.g., wherein the materialthermionically emits electrons and forms a conductive portion of the TECcircuit). The cathode can additionally or alternatively include one ormore additional layers (e.g., surface layers) configured to alter itsoptical properties, such as by enhancing above-gap photon emissionand/or depressing below-gap photon emission. Such layers are preferablythin (e.g., less than 1 nm, 1-3 nm, 3-10 nm, 10-30 nm, etc.), tominimize their interference with thermionically emitted electrons, butcan alternatively have any suitable thickness. Such layers can includemetal and/or semiconductor compounds (e.g., oxides, nitrides, etc.)and/or any other suitable materials. However, the cathode 200 canadditionally or alternatively include any other suitable elements in anysuitable arrangement.

1.3 System Arrangement.

The anode 100 and cathode 200 are preferably coupled, more preferablyfixed relative to each other. The anode 100 and cathode 200 arepreferably substantially parallel (e.g., a broad face of the anode 100substantially parallel a broad face of the cathode 200). The first sideof the anode preferably faces the cathode 200.

The anode 100 and cathode 200 preferably define a small gapcooperatively (e.g., between the broad faces). The gap can define aninter-electrode spacing in the range 100 nm-1 mm (preferably 1-10 μm),less than 100 nm, greater than 1 mm, and/or any other suitable spacing.The gap can be defined and/or maintained by spacers (e.g., separatingthe anode 100 and cathode 200), pockets (e.g, wherein a broad face ofthe anode and/or cathode is at the bottom of a pocket), and/or any othersuitable spacing elements. The system 10 preferably includes a vacuumenvironment and/or other isolated environment (e.g., isolated from anambient environment surrounding the system) within the gap. For example,the system 10 can include a system enclosure that functions (e.g.,alone; in cooperation with the cathode, anode, spacers, and/or othersystem elements; etc.) to enclose the gap and/or isolate it from theambient environment. The anode 100 and cathode 200 can be coupled bywafer bonding, mechanical fasteners, and/or any other suitable couplingelements.

The system 10 preferably includes electrical leads electrically couplingeach electrode to an electrical load (e.g., resistive load), such as ananode lead electrically connected to the anode electrical contact 160and a cathode lead electrically connected to a cathode electricalcontact (e.g., forming a thermionic energy converter configured togenerate an electrical power output using a thermal power input).

The system 10 can optionally include additional electrodes, such as oneor more electrodes (e.g., gate electrodes) arranged between the cathode200 and anode 100. For example, the system 10 can include anelectron-transmissive gate electrode (e.g., grid, electron-transparentmaterial, etc.) supported (e.g., from the enclosure, cathode, anode,etc.) between the cathode and anode, such as by electrically and/orthermally conductive and/or insulating supports. However, the system 10can additionally or alternatively include any other suitable componentsin any other suitable arrangement.

2. Method.

A method 300 for work function reduction and/or thermionic energyconversion can include inputting thermal energy to a system S310,illuminating an anode of the system S320, and/or extracting electricalpower from the system S330 (e.g., as shown in FIGS. 3A-3B). The method300 can be performed using the system 10 described above and/or anyother suitable system.

Inputting thermal energy S310 can function to maintain a cathode of thesystem at elevated temperature (e.g., 300-2500° C.). Electrons canthermionically emit from the cathode (e.g., toward the anode). Theelectrons preferably travel across a small vacuum gap separating thecathode and anode. The emission current is preferably high (e.g.,greater than a threshold current, such as 10 A/cm², 1 A/cm², 0.1 A/cm²,etc.), but can additionally or alternatively be moderate and/or low(e.g., less than the threshold current). S310 can optionally maintainthe anode at an elevated temperature. The anode temperature ispreferably in the range 250-350° C. (e.g., approximately 300° C.), butcan additionally or alternatively be greater than 350° C. (e.g.,350-450° C., 450-550° C., 550-650° C., 650-800° C., 800-1000° C.,greater than 1000° C.), less than 250° C. (e.g., an ambient temperaturesuch as 15-25° C., less than 15° C., 25-75° C., 75-150° C., 150-250°C.), and/or any other suitable temperature.

Illuminating the anode S320 can function to cause a photovoltage effect(e.g., as shown in FIGS. 4A-4B and/or FIG. 8). S320 preferably reducesthe anode work function (e.g., due to the photovoltage effect). Thereduction in anode work function due to the photovoltage effect (e.g.,the difference in anode work function with and without (or substantiallywithout) anode illumination, such as the difference between a dark workfunction and an illuminated work function) is preferably greater than athreshold amount (e.g., 25, 50, 75, 100, 150, 200, 250, 300, 350, 400,450, 500, 550, 600, 650, 700, 750, 800, 900, 1000, 1250, 1500, 0-25,10-50, 50-200, 100-300, 200-400, 300-500, 400-750, 700-1000, or1000-2000 meV, etc.), but can alternatively be a reduction of anysuitable amount.

The light illuminating the anode preferably includes photons with energygreater than a bandgap of a semiconductor of the anode (e.g., thephotons absorbed by and exciting band-to-band transitions in thesemiconductor). The light intensity is preferably 1-10 mW/cm² (e.g.,approximately 5 mW/cm²), but can additionally or alternatively be lessthan 1 mW/cm², greater than 10 mW/cm², and/or have any other suitableintensity. The light is preferably emitted by the cathode (e.g., thermalradiation from the cathode), but can additionally or alternativelyinclude light emitted (e.g., thermally emitted) by other elements of thesystem (e.g., enclosure, spacers, gate, plasma, etc.), ambient light,light from a dedicated light source (e.g., LED arranged near the anode),and/or any other suitable light.

Extracting electrical power S330 can function to use the power output bythe system. S330 preferably includes using the thermionic current fromthe system to power an electrical load (e.g., thermionically emittedelectrons travelling from the cathode to the anode, then through ananode electrical lead to an electrical load, and finally through acathode electrical lead back to the cathode).

In one embodiment, the method 300 preferably includes: heating thecathode; emitting light from the cathode (e.g., due to thermalradiation); absorbing light (e.g., the light emitted from the cathode)at the anode (e.g., at an n-type semiconductor of the anode), wherein awork function of the anode is preferably reduced in response toabsorbing the light; emitting electrons from the cathode (e.g.,thermionically); capturing electrons (e.g., the thermionically emittedelectrons) at the anode (e.g., at the electron capture layer, at thesemiconductor layers, etc.), preferably substantially concurrent withabsorbing the light; and/or providing all or some of the capturedelectrons as electrical power (e.g., wherein the electrons flow from theanode, through the electrical load, to the cathode).

However, the method 300 can additionally or alternatively include anyother suitable elements.

Although omitted for conciseness, the preferred embodiments includeevery combination and permutation of the various system components andthe various method processes. Furthermore, various processes of thepreferred method can be embodied and/or implemented at least in part asa machine configured to receive a computer-readable medium storingcomputer-readable instructions. The instructions are preferably executedby computer-executable components preferably integrated with the system.The computer-readable medium can be stored on any suitable computerreadable media such as RAMs, ROMs, flash memory, EEPROMs, opticaldevices (CD or DVD), hard drives, floppy drives, or any suitable device.The computer-executable component is preferably a general or applicationspecific processing subsystem, but any suitable dedicated hardwaredevice or hardware/firmware combination device can additionally oralternatively execute the instructions.

The FIGURES illustrate the architecture, functionality and operation ofpossible implementations of systems, methods and computer programproducts according to preferred embodiments, example configurations, andvariations thereof. In this regard, each block in the flowchart or blockdiagrams may represent a module, segment, step, or portion of code,which comprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block can occurout of the order noted in the FIGURES. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

We claim:
 1. A method for operating a thermionic energy converter (TEC),the method comprising: reducing a work function of an anode of the TEC,comprising: illuminating the anode with a plurality of photons; and atan n-type semiconductor of the anode, absorbing the plurality ofphotons; and substantially concurrent with reducing the work function,generating a thermionic current, comprising: at a cathode of the TEC,thermionically emitting a plurality of electrons; at the anode,substantially concurrent with absorbing the plurality of photons,capturing the plurality of electrons; and at the anode, providing theplurality of electrons as electrical power; wherein the anode comprises:the n-type semiconductor; a work function reduction layer; and anintermediary layer arranged between the n-type semiconductor and thework function reduction layer.
 2. The method of claim 1, wherein theintermediary layer comprises a transition metal oxide.
 3. The method ofclaim 2, wherein the transition metal oxide is titanium oxide.
 4. Themethod of claim 1, wherein the n-type semiconductor is selected from thegroup consisting of: n-type silicon, n-type silicon carbide, n-typegermanium, and an n-type III-V semiconductor.
 5. The method of claim 1,wherein illuminating the anode comprises, at the cathode, emitting theplurality of photons.
 6. The method of claim 1, wherein: reducing thework function of the anode comprises achieving an illuminated workfunction at the anode; and a dark work function of the anode whensubstantially not illuminated is greater than the illuminated workfunction by at least 50 meV.
 7. The method of claim 1, wherein the workfunction reduction layer comprises at least one of: an alkali metal andan alkaline earth metal.
 8. A method for operating a thermionic energyconverter (TEC), the method comprising: reducing a work function of ananode of the TEC, comprising: illuminating the anode with a plurality ofphotons; and at an n-type semiconductor of the anode, absorbing theplurality of photons; and substantially concurrent with reducing thework function, generating a thermionic current, comprising: at a cathodeof the TEC, thermionically emitting a plurality of electrons; at theanode, substantially concurrent with absorbing the plurality of photons,capturing the plurality of electrons; and at the anode, providing theplurality of electrons as electrical power; wherein the anode comprises:the n-type semiconductor; an electrical contact; and a layer opposingthe electrical contact across the n-type semiconductor, wherein thelayer does not comprise an alkali metal and does not comprise analkaline earth metal.
 9. The method of claim 8, wherein the layercomprises an oxide.
 10. The method of claim 8, wherein the anode furthercomprises a work function reduction layer, wherein the layer is arrangedbetween the n-type semiconductor and the work function reduction layer.11. The method of claim 8, wherein: reducing the work function of theanode comprises achieving an illuminated work function at the anode; anda dark work function of the anode when substantially not illuminated isgreater than the illuminated work function by at least 50 meV.